Conventionally, metal sheets such as tin free steel (TFS) sheets and aluminum sheets which are the materials for metal cans used as food cans have been painted in order to increase, for example, corrosion resistance, durability, and weather resistance. However, in the case of such painting techniques, there is a problem not only in that a baking process is complex but also in that a very long treatment time is needed and in that a large amount of solvent is discharged.
Therefore, in order to solve such problems, as an alternative to a painted metal sheet, a resin-coated metal sheet, which is manufactured by forming a laminated film of thermoplastic resin on a heated metal sheet, has been developed, and such a metal sheet is now widely used in industries mainly as a material for beverage cans.
However, in the case where a resin-coated metal sheet is used for food cans, since a portion of the contents strongly adheres to the inner surface of the container when the contents are taken out from the container, there is a problem in that it is difficult to take the contents out. Such a problem, which is closely related to the buying motivation of consumers, is a very important issue from the viewpoint of achieving the buying motivation of consumers. Nevertheless, in the case of conventional resin-coated metal sheets, very little consideration has been given to an improvement in the ease of taking contents out.
Therefore, the present inventors diligently conducted investigations in order to achieve a satisfactory content release property, and, as a result, found that it is possible to achieve good properties by adding a specific wax to a polyester resin so that the wax exists on the surface of the resin in the case of contents (such as a mixture of meat, egg, and carbohydrate) containing a large amount of fatty components, and filed Patent Literature 1.
In addition, the present inventors, from the results of subsequent investigations, made improvements in order to achieve an excellent content release property even in the case of contents (such as luncheon meat) having a high protein content and strong adhesiveness, and filed Patent Literature 2 and Patent Literature 3.